Improving green leafy vegetables cultivation with processing waste of horticultural products as source of potential biostimulants

In this work we are interested to improve growth and nutrient uptake of “Romaine” lettuce seedlings (Lactuca sativa ‘Longifolia’) in a floating system under greenhouse conditions. Potato peels and apple residual pulp (e.g. apple pomace: the residue after the extraction of apple juice) were employed as natural biostimulants. Large amounts of organic waste are produced from food processing industries. In particular, the processing waste of fruit and vegetables could be an excellent source of bioactive compounds perceived by consumers as low health risk substances. Skin of organic “Blue Salad” red potatoes and residual of organic “Fuji” apples after squeezing were minced, macerated in deionized water (500 g in 1 L) for 14 and 21 days, in the dark, at room temperature (RT). The aqueous extracts were filtered and properly diluted in water (to 1 or 10 mL L−1). Treatment solutions were sprayed in the morning onto leaves until run-off, every three days for 5 times. Along with plant growth, also the mineral content was significantly affected by the biostimulants. These results represent an interesting and promising possibility of using the waste products of apple and potato processing industry to obtain biostimulants capable of supporting the growth of agricultural productions, such as vegetables cultured in hydroponic system